Nuclear magnetic resonance (NMR) logging uses the NMR response of a geological formation surrounding a borehole to determine characteristics of fluids and gases in the formation, providing a continuous record along the length of the borehole. NMR logging exploits the magnetic moment of hydrogen, which is abundant in formation water and hydrocarbons. In many applications, the received NMR signal amplitude is proportional to the quantity of hydrogen nuclei present in the formation.
Generally, NMR tools operate by imposing a static magnetic field on a geological formation. This magnetic field is traditionally referred to as the “main magnetic field” or the “static field” as it is usually independent of time and is given the symbol B0. A second magnetic field, which varies in time, is also applied. This field is typically designated as B1 and is traditionally called the “radio frequency field”. It is turned on and off at different times, to create incremental pulses. This second, perturbing field is therefore usually applied perpendicular to the static field, B0. The perturbing field moves the orientation of the magnetization into the transverse (perpendicular) plane.
NMR logging may be used in determining the viscosity of oil. However, complications may arise when trying to determine heavy oil viscosity since the structure and nature of crude oil with heavy components gives rise to fast NMR relaxation.